Assessment of sediment/water contamination by in vivo transplantation of the cockles Cerastoderma glaucum from a non contaminated to a contaminated area by cadmium.

In this study the cockle Cerastoderma glaucum, a filter-feeding bivalve living in the upper layer of sediment was used to investigate the cadmium contamination at a heavily urbanised and industrial area, with a view to using them as an indicator of water/sediment contamination. To this end, cockles collected from indigenous population in a relatively uncontaminated site (Ras Ungha) were in vivo transplanted into sediment and water removed from cadmium contaminated site (El Hofra) for 45 days. The manipulative experiment was undertaken in order to examine the trace metal bioavailability in the contaminated area and to establish an analytical framework between the bioaccumulation of cadmium in the tissues and their biological effect in transplanted cockles. For this purpose, a range of sublethal stress biomarkers were selected on the basis of their potential to provide relevant information. Cadmium concentrations were determined in the sediment and in the soft tissue of the cockles from the two studied stations at time 0. Compared to the reference site, cadmium concentrations in the contaminated site were 53 higher in the sediment and 15 higher in the whole soft tissues. The variation of cadmium concentrations and biomarkers responses in transplanted cockles were determined as a function of exposure time. After 45 days' experience, cadmium concentrations increased by a factor of 5 compared with time 0. No significant change could be detected in controls. In the digestive gland of exposed cockles cadmium was mainly associated with the cytosolic fraction. The significant increase of cadmium concentration in the soluble fraction was followed by a significant increase in the concentration of the Sulphydryl-Containing Heat Stable Compounds (SCHSC) including metallothionein like proteins by approximately 86%. This is led to investigation into possible existence of an induction of MT-like proteins in relation to cadmium accumulated by exposed cockles. Transplanted cockles were also subjected to some stress effect consisting in significant inhibition of cholinesterase activity by 26.5% when compared with control cockles. Increased levels of malondialdehydes (MDA) following cadmium exposure have been also reported, suggesting that exposed cockles have been submitted to an oxidative stress probably due to the presence of high cadmium contamination in the sediment. Principal component analyses showed that cockles in vivo transplanted into cadmium contaminated sediment and water were standing out progressively from the control group as a function of exposure time. A clear separation of the transplanted cockles from their controls was observed after 45 days' experience. But, the transplanted cockles appeared not having yet reached the same characteristics as the resident cockles from the contaminated site.